During the manufacture or subsequent handling of PCBs, defects such as discontinuities (cracks) or unwanted continuities (shorts) may develop in or between circuit pathways and electronic components. It is necessary to do automated testing of PCBs both for manufacturing and maintenance purposes.
Testing of PCBs is becoming increasingly difficult and more expensive as the use of surface mount technology increases and as integrated circuits and PCBs become more complex and operate at higher frequencies. Conventional techniques for automated PCB testing are based on the idea of applying signals through a set of test pins and measuring output signals at other test pins. This method requires tight mechanical tolerances for the board layout, easily accessible test points, and restricts the frequency band at which a board can be tested (most of the test systems are limited to 100 MHz). The novel method presented here does not have these constraints because of its contactless nature. Another factor separating existing test techniques from this invention, is the contactless test system universality. The CTS does not need the custom setup of test pins and test patterns for the BUT, which make presently used test systems expensive and inaccessible to some complex circuit boards. Therefore, the applied CTS offers substantial advantages over existing test methods which utilize electrical contact.
Non-contact probes have been used for; measurements on high frequency microwave circuits. However, at frequencies below 1 GHz, the test is difficult due to the high bandwidth of the probes. Most recent advances of the test equipment industry have resulted in devices for the evaluation of electromagnetic compatibility (EMC) of PCB assemblies. However, these devices, in their present forms, provide only information about electromagnetic interference caused by the BUT, and can not be used for providing detailed information about the performance of the BUT. This invention is targeted at measuring detailed EMF for testing the quality of the BUT.
The previous patent by Goulette et at. (U.S. Pat. No. 5,006,788) described a similar method as that in this patent. But, the distinction between this patent and the previous one can be easily seen as follows. In the previous patent, in order to test a whole board, all the circuits or conducting elements on the board have to be connected to AC signal sources, therefore emitting the wanted electromagnetic field. To connect all the circuits (or conducting elements) on the board to active AC signals in the Goulette method is absolutely dependent on the circuit layout on the board and requires a specific complex connection setup for testing a whole board. Therefore, the testing method is not universal for testing all types of boards and may be only suitable for testing small, local areas of the board rather than a whole board.
The invention described here provides an universal and contactless method of testing for manufacturing faults and quality of any type of unpopulated or inactive populated printed circuit boards due to the use of the energizing plate. This method does not require any electrical contact with the BUT or a special test setup dependent on the board design layout, both of which are required by the previous invention.